The divergence experienced by a light beam propagating in a homogenous medium may be dramatically reduced in a multi-layered dielectric structure with appropriate choice of the materials and thicknesses of the layers. This effect is commonly used wherever guiding or confinement of the light beam is necessary, such as in optical fibers, solid-state optical waveguides or semiconductor lasers. The latter typically utilize a dielectric slab or rib/ridge waveguide structure to guide electromagnetic modes within the optical cavity resonator and to increase the overlap of the guided mode field with the laser active region. These guided modes are described as solutions of eigenvalue equations derived from Maxwell's equations with the proper set of boundary conditions given by the waveguide structure. A typical dielectric waveguide is based on refractive index contrast between a waveguide core (which includes the laser active region) and a pair of cladding regions; that is, the core, which has a relatively high refractive index, is sandwiched between cladding regions, which have a relatively lower refractive index. The ratio of the core/cladding refractive indices, the thickness of the core, and the polarization of the electromagnetic field determine the transverse dimension (the full width at half maximum (FWHM) of the mode intensity profile in a direction normal to the layers) and confinement factor .GAMMA. of the guided mode. .GAMMA. is in general defined as the fraction of the area under the guided mode intensity profile that overlaps the active region.
The transverse dimension of the confined modes is proportional to the effective wavelength of the radiation in the dielectric structure. Therefore, if we neglect the wavelength dispersion of the refractive indices, the propagation characteristics of the structure are preserved if the layer thicknesses are scaled linearly with the wavelength. For semiconductor lasers with emission wavelength in the mid-infrared (IR) or even far-IR, growth of very thick core and cladding layers is then required to efficiently confine the electromagnetic modes. For example, in the case of mid-IR (e.g., center wavelengths of 8-13 .mu.m) quantum cascade (QC) lasers in the AIInAs/GaInAs materials system, a waveguide based on primarily dielectric confinement, with an overlap factor of; say, .GAMMA..apprxeq.0.4, would require cladding layers about 6 to 8 .mu.m thick due to the relative weak refractive index contrast within the GaInAs/AIInAs materials system (i.e., at .lambda..apprxeq.10 .mu.m, n.sub.GaInAs =3.43 and n.sub.AIInAs =3.18, for undoped material). Of course, in the far-IR regime even thicker cladding layers (e.g., &gt;10-20 .mu.m) would be required.
The cladding layer thickness can be reduced by introducing a highly doped layer into the outermost part of the waveguide, with a doping concentration such that the plasma frequency approaches that of the optical mode. The resulting strong decrease of the real part of the refractive index, due to the anomalous dispersion, enhances the refractive index contrast, but it also introduces losses in the highly doped layer due to free carrier absorption. See, for example, three papers by C. Sirtori et al.: Applied Phys. Lett., Vol. 66, No. 24, pp. 3242-3244 (1995), Applied Phys. Lett., Vol. 69, No. 19, pp. 2810-2812 (1996), and IEEE J. Quantum Electr., Vol. 33, No. 1, pp. 89-93 (1997,) all of which are incorporated herein by reference. As pointed out in the first of these articles at page 3243, the highly doped layer plays a "crucial role" in suppressing the coupling between the lasing mode (propagating in a conventional dielectric waveguide) and the "high-loss plasmon mode" (that might otherwise have propagated along the metal contact interface). That is, the laser is designed to suppress, not support, the plasmon mode.